Increase in capacity of an optical fiber transmission system has been largely advanced by speeding up intensity modulation and increasing the number of multiplexed wavelengths. Recently, advancement in a digital signal processing technique has enabled transmission capacity to be further increased by using an existing optical fiber network, with a polarized light multiplexing technique and a multi-level phase modulation technique.
An optical fiber transmission system in which the multi-level phase modulation technique is adopted needs, in a receiver, a device called a hybrid mixer that converts a phase-modulated optical signal into an intensity-modulated optical signal. The hybrid mixer causes signal light propagated through an optical fiber to interfere with unmodulated local oscillation light (LO) having fixed amplitude and phase. The hybrid mixer causes the signal light to interfere with the LO to thereby convert phase change of the phase-modulated signal light into an intensity-modulated signal light.
The hybrid mixer is an optical circuit device that causes two light beams LO having phases different from each other by 90 degrees to interfere with two signal light beams having the same phase, respectively. To implement the hybrid mixer, an optical waveguide device having a superior optical characteristics is widely used. The hybrid mixer includes two optical waveguides (distribution optical waveguides) for each of distribution of the signal light beams and distribution of the light beams LO. The configuration of such a hybrid mixer is disclosed in the patent literature 1 (International Publication No. WO2012/086846).
There is a strong demand for smaller and lower-cost hybrid mixers, to implement a high-performance system with lower cost. For this reason, technical development has been made to implement, using complementary metal-oxide semiconductor (CMOS) process technology, a silicon optical waveguide which enables drastic downsizing compared with a glass optical waveguide widely used so far. The CMOS process technology is widely used in large scale integration (LSI) manufacturing.
Further, in relation to the present invention, the patent literature 2 (Japanese Patent Application Laid-Open Publication No. 2000-091319) describes a technique for forming a dummy pattern to achieve uniform etching on an entire wafer surface.